Here's How a Real Mechanic Checks a Used Car Before Buying

today I'm gonna show you some mechanics tricks for figuring out what's wrong with cars they're much easier to do than old methods and will keep you from wasting money on repairs that are needed no over the years I've had hundreds of people bringing their cars they've been to other mechanics they spent a whole bunch of money and when I look under the hood there's all kinds of parts have been replaced but the vehicle doesn't run right so I'm going to show you some tricks how to test them right in the first place and not just buy expensive parts on a whim that don't fix your car now the first thing to understand about internal combustion engines is they're basically air pumps they suck air in they burn gasoline they throw exhaust gas out there pumping air and to do that right they have to have the correct compression inside the engine the piston rings on the Pistons they seal the engine so it's tight and on the topside the valves have to seat right against the head or you'll lose compression and if you lose compression the car's not gonna run right with the car that might have a problem people can't figure out start with the basics see what the compression is cuz if the compression is bad and one or more cylinders you're wasting your money trying to fix other things you realize it's an engine problem that's the root of it all not a sensor or an alternator Sun no any old days a compression test was easy you just take out a spark plug your spark plug socket ratchet take it out well I'm screwing again in my plug then they come out it's best to have a magnetic one too it's got a magnet in it it's not hold it in place and won't fall out then you get a compression gauge and hook it up in this case just like an airline it's got a little clip and in the case you take out the one that's your spark plug which is this one and if you confuse get your own spark plug match it up you see the threads in a chair so it's gonna screw in just kind of stick so get some wd-40 spray it on the threads in the o-ring then screw it in simply screws in that's nice and tight then you get your gauge and snap it's seal beyond what the vehicle to start so can I count the fuse box go over here who won't we look in the top we can see there's a fuel injection fuse so we'll go here pull the fuel injection fuse out so it won't start yeah we go inside the car crank it over alone listen huh we go to the gauge and see what it says in this case it's 180 psi pretty good for an old engine it's got 240 thousand miles on it then you go and check them all four it's a four cylinder and compare if there's more than 15 20 percent difference between them are all really low the engine split why not then you got to decide you want to rebuild the engine or just get another car don't waste your money buying other parts but in many modern cars like this Lexus v6 it's very hard to do an actual compression test because now the front three or easy they're right here the plastic cover comes off but the back three cylinders they were under the intake manifold you have to pull the intake manifold off to do compression test of the back three cylinders that's gonna take hours it's a big job so I'm gonna show you how you can do a relative compression test works perfectly fine and you don't have to take anything apart in this case we're gonna do a relative compression test electronically you just need a meter and a low amp probe now you can use all kinds of meters I'm going to show you the cheap way to do it I bought this one just to see how the cheap ones working it works fine got in the Amazon it was like less than 80 bucks I have 800 and $1200 meters or you can get a big laptop oscilloscope you doing everyone just one that's less than 80 bucks works perfectly fine for doing a waveform analysis of it as you can see here's how you set it up the low amp probe you get the black round then put it on the black come here very simple and you take the redline and put it on the voltage in the amp one right here that's it and again we don't want the vehicle to start so we're gonna take the EFI fuse out pull it out a few as you can see it says right here EFI 15 so it shows you which fuse to rule then we turn on the current probe we want to put it on highest rating not the law one Nevada for ten millionths but the one millimeter 400 milliamps put it on the top one very simple then you turn the meter on we're going to put on millivolts cuz millivolts measures the smaller stuff and we push the auto display so instead of numbers it's now a graph then we get the lower amp probe we want to zero it because they need zero and you push the zero button then we just clamp it on the negative wire coming off the battery and now as we crank it off you can see the white banners you know support sound arises as you can see the patterns are pretty much perfect they're all pretty much the same thanks what this test does is show how much energy it takes to spin the engine for each cylinder as the starter spins the engine strains against it if you have one cylinder that dropped out that's got low pressure you'll see and this is a four cylinder you see three lines that were normal and at one line it'll be real short and it would go long long long short then you know you got a compression problem in his vehicle since the fuel injection system is disabled the only thing that's happening is the engine is pumping air if it takes a lot less to pump one of those the power usage is gonna go down so the oscilloscope will show you got power power power and then low power meaning one of the cylinders is bad you can go even further and get a fancy oscilloscope to have a trigger set it up by the number one and then you'd know exactly which cylinder is bad if you know a cylinder is bad then you decide maybe you're gonna get rid of that cop or if you're looking at a used car and you do the simple test don't buy the car it's very easy uses your testing I used car you just take the efi cues up clamp the meter on to the oscilloscope and crank it over look at the wave pattern if it's got a bunch of good ones in a bad one a bunch of good ones in a bad one you know it's got a bad cylinder now it's quick and easy on any car but econo luck or let's say you got a 12 cylinder engine it's got spark boats hidden all over the place that can take a day to get too high simply plug it in crank it over watch the lines it's a good relative compression test that anybody can use and I've done this on people's cars that have spent a fortune and other mechanics putting on air flow sensors alternators fuel injectors they spend thousands of dollars and all I do is pull out the fuse for the EFI do this test and find out that they have one or more cylinders that have low pressure and say hey you wasted all their money now you got to decide you want to rebuild the engine do you want to put in a used danger or you just want to get rid of the car knowledge is power in this case it's very easy to get this knowledge now the second test that I'm gonna show you it's testing fuel pumps we used to use fuel pump gauges they had a little Schrader valve easy to test but not anymore on each Toyotas you got to take a line off have a special adapter fuel gets all in a place it's a mess you can also do this by doing inductive testing watch this but you go to the fuse box here look on the bottom and then ya Phibes over here pull the efi fuse oh and get the little probe and we plug it in so it's nice and tight huh now the only thing that's going to be different from the other test is the other test we have checked in the cranking of the engine that's a lot of power so instead of having it at the high one we're gonna put it on the wall one here that's just one millivolt for ten millionths and we turn the meter on we still use the millivolts and we don't want the reading we want a graph so here we go and now we're gonna start the car start up and here's the better it's a pretty consistent pad and we can hold it man Wow I want you past there you can see it's a pretty consistent pattern if you find that it's got drop-offs in one place or another that would mean that the pump was starting to fail because it's just like measuring the cranking of the engine only it's measuring the spinning of the electric fuel pump that's inside the gas tank and if it had a certain amount of power can then every once awhile it either spiked up using way too much power or spiked down not using enough you know that it either had a short to power or a short to ground if the pump was wearing out not Toyota's aren't that bad to change the fuel pump ahead you take the back seat out there's a little access panel you take it out and replace it but they're very expensive pumps you don't want to guess with it you can do this simple test like that you know I'm making a gasoline mass room gasoline getting over by disconnecting stuff and putting gauges on and then of course you'd have to buy new o-rings because you can't use the o-rings when you take the fuel system off and plug it to the gauge then oring is there compromises needing you all rinse this is a clean not messy way to check a fuel pump especially when a vehicle is cold a lot of cost us bring me a car let's say it starts bad when it's cold so look that's dust up when it's cold and I have really bad lines when it's cold that when it warms up the pump warms up and works about it but since it doesn't when it's cold and I test it when it's cold I can see aha the pump is starting to wear out when it's cold a lot of machines when they're cold they're harder to work any warm-up they work better it shows you the pumps on its way out you know change it sooner and later very important tests on your car to see if your engines worn out to see if your fuel pump is wearing out by using one of these digital graphene meters and since it's mechanic Monday I'm gonna give away one of these things to have a chance to win just place a clean non offensive comment on the YouTube comments below and a winner will be chosen randomly by computers to get one of these cool digital graphene meters for working on their car or maybe even a neighbor's car so if you never want to miss another one of my new Chi repair videos remembering to ring that Bell [Music]

Channel: Scotty Kilmer

Views: 1,058,924

Keywords: engine compression, engine compression test, how to check engine compression, auto, buy a used car, buying a car, buying a used car, car, car advice, car buying, car diy, car inspection, car repair, car review, cars, diy, how to, how to buy a car, how to buy a used car, how to check a used car, how to inspect a used car for purchase, mechanic, repair, used car, used car buying, used cars, how to do compression test, this test saved me thousands when buying a used car

Id: RLOpo71JtIU

Length: 10min 48sec (648 seconds)

Published: Mon Apr 06 2020